Ion exchange membranes are widely used in electro-membrane processes, including, for example, electrodialysis, Donnan dialysis and diffusion dialysis. Efficient electro-membrane processes require ion exchange membranes with a combination of properties such as high permselectivity, low resistance, high chemical stability and good shape stability, also referred to as controllable swelling.
Ion exchange membranes may be produced by incorporation of ion exchange material into an inert matrix, obtaining heterogeneous membranes. Alternatively, homogeneous membranes may be prepared, that consist of a single polymer or polymer mixtures carrying ionizable groups. Surface coating of ion exchange membranes is sometimes used to achieve univalent ion selectivity (see for example Toshikatsu Sata (2000), Journal of Membrane Science, Vol. 167 pp. 1).
Anion exchange membranes (AEM) are those that reject certain positively charged solutes and allow neutral molecules or negatively charged solutes to pass the membrane. For example, ionomers with anion exchange groups may be used for the preparation of the membrane. The anion exchange groups may be chosen, for example, from quaternary ammonium, phosphonium and sulfonium.
Ionomers with anion exchange groups may be made by derivatizing aromatic condensation polymers, such as polysulfone, polyethersulfone and polyetherketone, to form e.g. halomethylated polymers, which are converted to quaternary ammonium, phosphonium and sulfonium derivatives. Examples of such ionomers include halomethylated engineering plastics, such as bromomethylated polysulfone or polyethersulfone, which may be reacted with tertiary amines and multifunctional amines to introduce crosslinking (Kedem and Warshaysky, U.S. Pat. No. 5,288,385). These membranes are not considered to be cost effective, as halomethylation of the above polymers is very expensive and the resulting ionomers swell at the ion exchange capacities needed to achieve good conductivity.
Other examples of homogeneous anion exchange membranes include insoluble polymer sheets, such as crosslinked polystyrene that are halomethylated and subsequently aminated to give quaternary amine groups, and films comprising partly aminated poly-4-vinylpyridine crosslinked with dibromo or chloro alkanes, which also quaternizes the remaining pyridine groups. These membranes cannot achieve a combination of good selectivity and low resistance without substantial swelling. In addition, when dry, the membranes are mechanically weak and brittle. Thus, most such membranes are made in combination with a net or porous support, which is embedded in the final polymer film. However, the supported dry membranes are still brittle when dry and increase in length/width when wetted.
Crosslinking is an important aspect of anion exchange membranes. For example, a crosslinked structure obtained by reacting a chloromethylated product of a polysulfone type polymer with a polyamine has been proposed (JP Patent No. JP-A-2-68146), and an anion exchange membrane having such a crosslinked structure and characterized by good corrosion resistance has also been proposed (JP Patent Nos. JP-A-6-80799, JP-A-6-172559 and JP-A-6-271688).
Other examples of crosslinked membranes include the membranes described in U.S. Pat. No. 6,780,893, which discloses, inter alia, a process for producing an anion exchange membrane, which comprises mixing from 25 to 95 mass % of a polymer having anion exchange groups or active groups convertible to anion exchange groups (“polymer 1”), and from 5 to 75 mass % of a polymer having no anion exchange groups or no active groups convertible to anion exchange groups (“polymer 2”), and forming the obtained composition into a membrane, said process including a step of cross-linking an aromatic ring of a repeating unit constituting polymer 1 with an aromatic ring of another repeating unit constituting polymer 1 or with a cross-linkable site of polymer 2, and in a case where polymer 1 is a polymer having active groups convertible to anion exchange groups, a step of converting the active groups to anion exchange groups.
U.S. Pat. No. 6,780,893 describes, inter alia, anion exchange membranes which are based on haloalkylated aromatic polymers, such as haloalkylated polysulfone, polyetherketone and polyphenylene oxide type polymers.
Crosslinking described in U.S. Pat. No. 6,780,893 may be brought about by a primary or secondary amine compound such as ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylenepentamine, polyethyleneimine or phenylenedianilne, or N,N,N′,N′-tetramethyldiaminomethane, N,N,N′,N′-tetramethyl-1,2-diaminoethane, N,N,N′,N′-tetramethyl-1,3-diaminopropane, N,N,N′,N′-tetramethyl-1,6-diaminohexane, N,N,N′,N′-tetramethylbenzidine, N,N,N′,N′-tetramethyl-4,4′-diaminodiphenylmethane, polyvinyl pyridine or a primary or secondary aminated product from polychloromethylstyrene. Alternatively, if polymer 1 has an alkyl repeating unit with Cl, Br, I or a hydroxyl group, the cross-linking reaction may be carried out by heat treatment or a Friedel-Crafts reaction, and then the remaining substituents are reacted with an amine to introduce anion exchange groups. In another method, substituents are reacted with a polyamine to carry out introduction of anion exchange groups and cross-linking at the same time.
The above approach is considered to be expensive because of the need for halomethylation and multistep process including crosslinking. In addition, high ion exchange capacity would be needed in order to achieve good conductivity, which would cause high dimensional swelling.
In general, known anion exchange membranes are either relatively expensive, as their preparation involves the process of halomethylation, and/or cannot achieve the necessary conductivity and selectivity without substantial dimensional swelling. Membranes which swell also have a tendency for crack formation when drying out in any system.
Fouling of anion exchange membranes is another major difficulty. For efficient electro-processes, the surface of the anion exchange membrane should minimize polarization, water splitting and adsorption of contaminants. Polarization is particularly high in the case of heterogeneous anion exchange membranes.
Attempts have been made to overcome the very high polarization of heterogeneous anion exchange membranes. Examples include the addition of a hydrophilic and conducting surface layer, as described in Kedem et al. (1998), “Low polarisation ED membranes”, Desalination, Vol. 118 and Oren et al. (2003) Modified Environmental Engineering Science, Vol. 19(6), pp. 512. Polarization could be reduced by using this modification relative to the high polarization obtained with heterogeneous polyethylene based membranes. However, such coating could not overcome the problem of fouling.
Polarization in electrodialysis processes may also be overcome by using coated spacers in ED unit that separate anion exchange membranes from cation exchange membranes. For example, U.S. Pat. No. 6,090,258 discloses a polymeric netting for use as an ion-conducting spacer in an electrodialysis stack having charged groups incorporated in a polymeric coating applied thereto, imparting to the spacer an average ion exchange capacity of at least 0.25 meq/gr.
Thus, there still remains a need for cost effective anion exchange membranes, possessing a combination of good electrochemical performance and mechanical strength, as well as low degree of dimensional swelling, fouling and polarization.